Print heads of the type described are well known. For example, U.S. Pat. No. 4,364,064 describes a fiber optic print head in which an in-line array of optic fibers are held in grooves etched into a silicon plate. Each optical fiber has an independently modulatable laser diode optically coupled to its input end and the head is positioned with the fiber output ends closely adjacent to the photoreceptor surface so that, as the head is translated along the surface, the several modulated light beams exiting the fibers simultaneously write a corresponding number of parallel image scan lines.
The need for high resolution image writing, e.g. 2400-2800 pixels per inch, dictates that the center-to-center spacing between adjacent beam spots on the photoreceptor surface be very close. For example, a resolution of 2800 pixels per inch requires a spacing of 9.07 microns. Conventional, unmodified single mode fibers are presently available with an 80 micron diameter and can be mounted on a substrate with a center-to-center spacing of 90 microns. If the light beams are projected directly onto the photoreceptor, as is described in the above patent, such a head would write a comparatively low resolution image of about 280 pixels per inch in the cross-scan direction. Multi-mode fibers are available with with a diameter of 50 microns but even these would give a relatively low cross-scan resolution on the order of only 420 pixels per inch.
Various techniques for achieving the desired close spacing are known. One such technique involves tilting the head at an angle to the writing direction. However, for reasonable tilt angles, the fibers must be etched to reduce the diameter so as to increase the packing density of the fibers in the head. Fiber etching processes increase the cost of the head unduly and can introduce unwanted centration errors in the fibers. Moreover, only single mode fibers, which have thick external cladding, can be etched. Multi-mode fibers, which have a much larger numerical aperture and very thin cladding, cannot be effectively etched.
In addition to direct projection of the beams onto the photoreceptor, it is also known to interpose an optical imaging system between the print head and the photoreceptor to increase the depth of field and allow the head to be spaced safely away from the moving photoreceptor surface. By using imaging optics with a fractional magnification, the center-to-center spacing of the spots can be reduced to achieve the desired spacing. Unfortunately, however, this would also result in reduction of the spot size to an unacceptable level. For single mode fibers operating with light beams near infrared wavelength, the spot size at the end of the fibers is typically on the order of 5 to 6 microns in diameter at the l/e.sup.2 point while the spot size required at the photoreceptor surface is on the order of 19 microns. If an optical system with a 0.1.times. magnification is used to bring a 90 micron spacing down to 9 microns, the spot size would also be reduced by a factor of 0.1.times. on the photoreceptor surface.
It is therefore an object of the invention to provide scanning apparatus employing a non-contact print head that is economical to produce.
It is another object of the invention to provide scanning apparatus using an in-line fiber optic print head utilizing readily available optical fibers that do not need to be etched to achieve desired center-to-center spacing for high resolution image writing.
It is another object of the invention to provide a non-contact optical print head that utilizes imaging optics both to improve the depth of field of the print head so as to increase the spacing between the head and the target surface and to achieve desired image resolution.
It is another object of the invention to provide a non-contact optical print head of the type described in which spot size and spacing between spots on a photoreceptor target surface can be set independent of each other.